Pixel Strike 3D

I don't know why i got downvoted but thanks to that, the 1st solution appearing is wrong. The achievement is NOT Cumulative+ like OP said.

This is Cumulative-, as confirmed in the Discord server. Confirmed by @Endorja and myself as we both spent Coins at some point in our progress, and both got our achievement at 100.000 flat.
The flags are wrong, you NEED to have 100k Coins in your counter at the same time, using some will waste them coins. Flag has been changed.

-You need 100.000 Gold in your bank, at the same time, spend some and you lose progress.

-Play as much as you can on Fridays, every Friday is Double Coins for 24h (you can also buy double the content for the same price in the shop)

-Buy MVP as early as you can ($1.99USD i believe) which gives you double daily tasks rewards and will also grant you a perma 20% Coins rewards bonus, meaning you now effectively have to get 80k Coins instead of 100k (just effectively, compared to a non-MVP user. You still need 100k) As of writing this, this may not work and MVP Benefits may not work at all

-Do you daily tasks, with MVP you get 40 Coins per task, you usually have 2 tasks related to 1 Weapon HS/Kills and 1 Perk then 2 Game Modes. So make a class to get both Weapon/Perk tasks done ASAP and restart your game, this will reroll your tasks. Faster to do this until you have an other weapon task instead of for example a "Win 4 CTF games" task.

-Save up your Credits and buy ELITE Status as much as you can as it will give you a 150% Boost for all your Coins earnings (minus daily tasks).

-Finally, get kills and level up, these are the only way to get coins (besides 4 daily tasks and spending real money). The way they calculate how many coins you get per kill seem random to me, sometimes 50 Kills gets me 60 Coins, sometimes 45, no clue...
The best way to get kills is play in Quick Matches since Crossplay is turned off and everyone there is.... new... yes, new is the right word... everything besides Infected and Showdown are good but all FFA modes are better to farm kills.
CTF is also good, but let's say you get 100 kills every full CTF game (15min), in 3 FFA matches you get 150 kills for as much time spent so imo, Quick Match is the best option for both Coins and XP.

Edit: i also recommend the FFA/TDM/Gun Game Playlist in Competitive section, that way you dodge all the Sniper Modes but more importantly, no more Moshpit, Infected and Showdown which are a huge waste of time.
Be careful as the lobbies can get sweatier, you are no console locked anymore, this is 100% Crossplay. Quick reminder, i have about 250h in the game, and i met about 5 cheaters, if you get rolled by a mobile/PC player, chances are it is NOT a cheater, report them if you want, but don't lose your mind over it, they simply played a lot longer, have macros at their disposal (they are allowed by the devs), Mobile have auto-jump when shooting, PC have huge fps advantage, etc.. but there barely is anyone cheating in those playlist.

The original solution is good enough of an explanation but i wanted to add my take on this as well.

This is Cumulative-, as confirmed in the Discord server, the flags are wrong, you NEED to have 100k Coins in your counter at the same time, using some will waste them coins.

-Play as much as you can on Fridays, every Friday is Double Coins for 24h (you can also buy double the content for the same price in the shop)

-Buy MVP as early as you can ($1.99USD i believe) which gives you double daily tasks rewards and will also grant you a perma 20% Coins rewards bonus, meaning you now effectively have to get 80k Coins instead of 100k (just effectively, compared to a non-MVP user, you still need 100k) As of writing this, this may not work and MVP Benefits may not work at all

-Do you daily tasks, with MVP you get 40 Coins per task, you usually have 2 tasks related to 1 Weapon HS/Kills and 1 Perk then 2 Game Modes. So make a class to get both Weapon/Perk tasks done ASAP and restart your game, this will reroll your tasks. Faster to do this until you have an other weapon task instead of for example a "Win 4 CTF games" task.

-Save up your Credits and buy ELITE Status as much as you can as it will give you a 150% Boost for all your Coins earnings (minus daily tasks).

-Finally, get kills and level up, these are the only way to get coins (besides 4 daily tasks and spending real money). The way they calculate how many coins you get per kill seem random to me, sometimes 50 Kills gets me 60 Coins, sometimes 45, no clue...
The best way to get kills is play in Quick Matches since Crossplay is turned off and everyone there is.... new... yes, new is the right word... everything besides Infected and Showdown are good but all FFA modes are better to farm kills.
CTF is also good, but let's say you get 100 kills every full CTF game (15min), in 3 FFA matches you get 150 kills for as much time spent so imo, Quick Match is the best option for both Coins and XP.

This is cumulative. The best way to get this achievement is by either wasting 500 dollars and get this or buy elite with credits and hop on Quick Play/Core/CTF and try to get as much kills as you can. I usually use Battle Rifle/Special Forces and Desert Eagle.